Many articles, such as parts or finished goods, have paints or finishes applied to them as part of their manufacturing process. How well a paint or finish adheres to the surface of an article to which it is applied depends upon several factors, including: the paint or finish composition, the surface composition, the conditioning of the surface prior to application of paint or finish, the presence of an undercoat or intercoat prior to application of an outer coat, the temperature and humidity during application, the method of curing or drying of the paint or finish, and the time allowed for the paint or finish to cure or dry. However, even with rigidly controlled manufacturing processes, variations in paint and finish adhesion occur.
To determine the adhesion to the article surface, organic coatings are sometimes tested using a material or device that is pressed against the coating surface for a certain duration and then removed. For example, a material such as a pressure sensitive adhesive tape is secured to the coating and then removed. If any coating is removed with the tape, then the adhesion of the coating is deemed to have failed the test. There are several variations of these adhesion tests, including rollers that press the tape to the coating at a predetermined pressure, water saturated materials that pre-soak the coating prior to application of the tape, and scribes that physically score the coating prior to application of the tape. The water and scoring facilitate the tape's ability to remove coating from the underlying surface and make it easier to assess if the coating is adhering poorly.
In another example, a metal dolly having a flat circular surface is secured to the coating using an adhesive. Once the adhesive has cured, the dolly is pulled at various pressures until the dolly pulls the coating from the article or the adhesive detaches from the dolly or coating. In one variation of this method, a cutting tool is used to drill a channel through the coating to the article's surface around the dolly to isolate the test area. These test methods are destructive to the coating. Further, the test methods can damage the article under test if any scoring or drilling penetrates below the depth of the coating into the article itself. Therefore, these tests are typically performed on sample articles rather than on the actual goods to be sold. For example, a sample article from a batch of identically manufactured articles is selected at random from the batch to be the representative article and destructively tested using the methods described above. The assumption is that each article from the batch will have nearly identical coatings to the representative article because they were manufactured at approximately the same time under similar conditions.
There are several disadvantages to destructive test methods. First, if the articles under test are large or expensive items, such as aircraft bodies, it may be impractical to use destructive test methods. Second, even if the articles are manufactured under identical conditions, there may be differences in coating adhesion from article to article. Third, a false negative during testing of the representative article can result in disqualification of perfectly good articles in the batch, or at a minimum require further destructive testing of additional articles. Fourth, destructive testing of representative articles adds to overall costs. Fifth, because only representative articles are tested, not every article that is manufactured can be tested before being sold or used. The present test methods do not address these disadvantages. Therefore, there is a need to develop a non-destructive test method for testing the adhesion of coatings to articles.